Add (deps (library x)) to depend on an installed library in rules

Right now, most of the dune constructs are "referentially transparents", e.g. they don't care if the stanzas are referring to objects installed with opam or are which present in the current workspace. This includes most of the library and executable stanzas (with (libraries ...)), variables (e.g. %{lib:...}), etc.

However it doesn't seem possible to do the same thing in rules. (deps (package x)) breaks this: it requires x to be present in the workspace and it will fail with Error: No rule found for alias .x if x is installed with opam.

I understsand that dune doesn't know anything about opam packages, and that requiring an opam package to be installed is a bit weird ; So instead I'm proposing to add a (deps (library x)) stanza instead, with the same semantics as (libraries ..) appearing in libraries/executables. WDYT?

Start by comparing (deps (package x)) in rules with the (libraries ...) semantics for library and executable stanzas, reproducing the failure when x is installed with opam rather than present in the workspace. Done means rules can express (deps (library x)) and resolve an installed library with the proposed referentially transparent behavior.